Commercial trucks and trailers have air brake systems, the most common of which function by using compressed air to operate a brake chamber on each wheel, which puts pressure on a pushrod that rotates a camshaft, applying an outward motion of the brake shoes against the brake drum. Each wheel can have a difference in pressure applied to the drums for a variety of reasons, such as brake adjustment, valve crack pressures, air leaks, worn or sticky brake components, etc. Uneven braking can be dangerous, and will cause the brakes to wear out prematurely. With a tractor/trailer combination it can result in a jack-knife situation if the braking isn't evenly balanced between the truck and trailer, but the driver has no way of knowing if his brakes are all balanced.
Commercial truck and trailer manufacturers are currently required to build the units with Anti-Lock Braking Systems (ABS), which works by using sensors to determine if a wheel is locking up on slippery surfaces, and reduces the braking pressure on that wheel to help maintain control of the vehicle. A vehicle could potentially be operating with several wheels out of adjustment or other mechanical issues, but in a lockup situation the ABS sensors may actually reduce the braking of the adjusted wheels.